Pioneering work on fibre bundle lasers was undertaken by the inventor at the Royal Radar Establishment, UK in 1963 when laser light was propogated through optical fibre bundles that were commercially available at that time. The early goal was to use bundles of coherently packed optical fibres to inter-connect laser rod media to a flexible membrane transmitting aperture of an early laser radar system. However, these experiments showed the complete unsuitability of early optical fibre bundles for the transmission of laser light. With the advent of single mode optical fibres during the 1970's, the situation was reconsidered and some details were published in 1979 (Hughes and Ghatak Applied Optics, USA Vol. 18, No. 13, 2098, issued July 1979) with the key elements being classified by the US and UK patent offices in 1984. The first commercially orientated invention that originated from this pioneering effort to develop phased-array, fibre bundle based lasers was described in a patent format (Hughes, U.S. Pat. No. 4,682,335 issued Jul. 21, 1987).
This invention covers an important aspect of the development of phased-locked fibre laser bundles in that it provides for the switching of a looped fibre laser bundle laser oscillator via the modularisation of the excitation light entering via the end face of a laser oscillator system which can be side excited to lasing threshold. In the case when the invention is used as a laser pulse amplifier, its end faced pulsed excitation can be of a travelling wave format to avoid selfoscillation of the said amplifier, a process which leads to the depletion of the stored energy with the excited amplifier prior to the passage of the laser pulse to be amplified.
The invention is essentially a glass laser in which the severe operating problems experienced with solid rod and slab glass laser media are avoided. Firstly, the low thermal conductivity of glass is overcome by using very thin glass fibres which can be easily cooled and also by using glass fibres in relatively long lengths so that the heat loading over the total gain path can be minimised. Secondly, the self-focusing problems caused by the radiation induced self-collapse of the laser beam within a laser medium is avoided because the thickness of the fibre core is comparable to the wavelength of the laser light undergoing amplification.
When used as an amplifier, the invention can be placed wholly or partly in a magnetic field which allows for the rotation of the plane of polarization of the laser light propogating through the fibre cores via the Faraday effect. Since the Faraday rotation effect is always in the same direction, it is an effective technique for rotating the plane of polarization in the present invention. Such a rotation of the plane of polarization of the input laser beam allows for a series of amplifier configurations of the invention to be optically coupled to form an amplifier chain. Both the oscillator and amplifier configurations of the invention are scaleable to high continuous and mean power levels respectively.